A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the ongoing efforts for attaining higher resolutions at smaller imaging dimensions, in lithographic systems, projection systems are used that have numerical apertures that are well above 0.8. These systems tend to be very bulky and wide in dimensions, in particular near the lower surface of the system where the radiation (light) exits the projection system for illuminating a substrate. Furthermore, these high numerical apertures have a working distance that is usually very small (only a few millimeters), which may cause the projection system to be placed very close to the wafer surface. As a result of this, in practical setups, there may be little room provided for arrangements that are operative in the area between the projection system and the substrate. One of these arrangements that are operative in the above indicated area is an air shower system that is present to condition the “air” (which is usually a very fine conditioned composition of gases) in this area. This conditioning, among others, is helps to define a stable gas environment so that interferometric measurement beams may be unhindered by refractive index fluctuations. This helps to obtain reliable (sub) nanometer measurements of the position of the substrate in relation to the projection system, so that a pattern may be reliably imaged at a predefined position of the substrate.
These bulkier projection systems and closer working distances may make it difficult to position the air shower system in such a way that this area as a whole is sufficiently reached. In particular, there may not be enough room to place an air shower system in such a way that the gas flow is sufficiently able to condition the volume in which the substrate holder is moved below the projection system. In this volume, the interferometric measurement beams extend to measure the position of the substrate. Generally, this volume may be bounded by the projection system, for example, the exit plane of the lower lens in the projection system. This volume may also extend below a mirror block that is used in determining the z-height of the substrate to be illuminated (Z-mirror).
Current setups for air shower systems generally have two major directions of flow: at a distance from the lens, the gas flows perpendicular to the longitudinal axis of an elongated volume defined by the path of the interferometric measurement beams, and closer to the lens center, the gas flows substantially at a small angle relative to the longitudinal axis. However, in the transitional region extending between these major flow directions, the flow is generally undetermined. This transitional region typically has a wide extension in the elongated volume it is attempting to condition so that in practice, large portions of the elongated volume may suffer from sub-optimal conditioning conditions.